1. Field of the Invention
The present invention relates generally to lasers, and particularly to external cavity lasers for use as a transmitter in optical communications.
2. Technical Background
In dense wavelength division multiplexing (DWDM) system applications, the transmitter wavelength has to be locked to one of the International Telephone Union (ITU) standard wavelengths of an ITU grid to meet crosstalk specification and ensure reliable operation of the system over its lifetime (about 25 years). The lasing wavelength of a free running commercial distributed feedback (DFB) laser, determined by its built-in DFB grating and refractive index of the semiconductor waveguide, changes with temperature at a rate of 0.1 nm/.degree. C. FIG. 11 shows a wavelength-locked DFB laser demonstrated by Nortel Technology as described in the article by B. Villeneuve, H. B. Kim, M. Cyr and D. Gariepy, "A compact wavelength stabilization scheme for telecommunication transmitter," Digest of the LEOS Summer Topical Meetings, WDM Components Technology, WD2, 19-20, Aug. 13-15, 1997, Montreal, Quebec, Canada. A slightly diverging beam of laser light 112 transmitted through a Fabry Perot filter or a single-cavity multilayer dielectric filter 114 is detected by two closely spaced photodetectors 116 acting as apertures. The photodetectors 116 are equally spaced from the centerline of a semiconductor source laser 118. Each photodetector 116 captures a different but overlapping center portion of the total solid angle emitted by the divergent laser light source, as the filter 114 is aligned to control and monitor the transmission wavelengths. Two different spectral responses, offset in wavelengths according to their angular difference, are produced as shown in FIG. 12. The difference or discrimination signal 222 is used by an operational amplifier 220 to control a heat sink temperature to lock the lasing wavelength to the ITU wavelength or center frequency .lambda..sub.0.
FIG. 12 shows the ideal case where the wavelength offset between the two responses is roughly equal to their effective bandwidths such that the center frequency is centered at the ITU wavelength. However, to reduce cost, it is desirable to eliminate the extra external feedback parts of the operational amplifier 220 and photodiodes 116 needed for wavelength discrimination in this type of temperature control of a wavelenth-locked laser while maintaining or improving temperature reliability.
Filter-locked external cavity lasers as shown in FIGS. 13 and 14 have recently been proposed and demonstrated. These lasers do not need the feedback control to monitor wavelengths since the center wavelength of its filters, made of a dielectric material, such as the fiber grating and the multilayer dielectric filter, has been demonstrated to be much less sensitive to temperature (&lt;0.005 nm/.degree. C.) than that of the semiconductor grating filter used in the DFB laser. A reflective Bragg grating written into the fiber establishes the precise lasing wavelength. One of the frequencies of the ITU grid is selected for the Bragg grating. The advantage of writing the frequency into the silica fiber is that the silica has a small coefficient of thermal expansion (about 5.times.10.sup.-7 /.degree. C.) and the resonant Bragg frequency changes can be made negligible by temperature compensation.
As seen in FIG. 13 and described in U.S. Pat. No. 5,844,926, a semiconductor laser diode chip 118 is provided with an anti-reflection (AR) coating 26 on one end facet 132 to which is optically coupled a length of optical fiber pigtail 134 in which there is a Bragg grating reflector 136 defining one end of a laser optical cavity whose other end is provided by the reflecting end facet 138 of the laser chip remote from the AR coated end facet. This Bragg grating reflector thus provide a means of locking the laser frequency.
Instead of using fiber, air can be substituted in the external cavity of FIG. 14. Here and described in U.S. Pat. No. 5,434,874 and U.S. Pat. No. 5,870,417, a gain medium, such as the semiconductor (laser chip) 118 has both front 138 and back 132 facets, as in FIG. 13, where the back facet 132 has the anti-reflection coating 26. Light 142 from the laser chip passes through the back facet 132 into an external air cavity. The cavity contains a tuning element 162, such as a prism, mirror, filter, or grating, that reflects specific laser wavelengths back into the laser chip 118. This round-trip light action 142 causes the laser to output selectable wavelengths 62 through the front facet 138. Thus, the wavelength of light output 62 from the front 138 facet of the laser chip can be controlled by changing the angle of the grating, filter or other tuning element 162. The cavity also contains a collimating lens 144 which directs light emitted from the rear facet 132 of the chip onto the grating, filter, or other tuning element 162.
However, due to its long external cavity, the filter-locked laser can not be directly modulated at a high bit rate. The 3-dB modulation bandwidth decreases as the external passive cavity length increases as seen in FIG. 9. For example, the direct modulation bandwidth of a semiconductor laser with a 300 .mu.m cavity length is about 10 GHz. Therefore, it is difficult to directly modulate a filter-locked laser at a rate of 2.5 Gbit/sec and beyond since the external passive cavity length is in the order of 1 cm or longer. Moreover, the direct modulation response at the frequency (peak frequency) corresponding to the round trip time is significantly enhanced as shown in FIG. 15. The peak frequency as a function of the external passive cavity length is shown in FIG. 10. If the peak frequency is close to one of the harmonic frequencies of the signal, the signal will be distorted. An external modulator is thus needed for high speed modulation. Additionally, there is at least a cost saving reason to integrate an external cavity laser with an external modulator.